When ath9k was switched over to use the mac80211 intermediate queues,
node cleanup now drains the mac80211 queues. However, this call path is
not protected by rcu_read_lock() as it was previously entirely internal
to the driver which uses its own locking.

This leads to a possible rcu_dereference() without holding
rcu_read_lock(); but only if a station is cleaned up while having
packets queued on the TXQ. Fix this by adding the rcu_read_lock() to the
caller in ath9k.

Fixes: 50f08edf9809 ("ath9k: Switch to using mac80211 intermediate software 
Cc: sta...@vger.kernel.org
Reported-by: Ben Greear <gree...@candelatech.com>
Signed-off-by: Toke Høiland-Jørgensen <t...@toke.dk>
 drivers/net/wireless/ath/ath9k/xmit.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/wireless/ath/ath9k/xmit.c 
index 396bf05c6bf6..d8b041f48ca8 100644
--- a/drivers/net/wireless/ath/ath9k/xmit.c
+++ b/drivers/net/wireless/ath/ath9k/xmit.c
@@ -2892,6 +2892,8 @@ void ath_tx_node_cleanup(struct ath_softc *sc, struct 
ath_node *an)
        struct ath_txq *txq;
        int tidno;
+       rcu_read_lock();
        for (tidno = 0; tidno < IEEE80211_NUM_TIDS; tidno++) {
                tid = ath_node_to_tid(an, tidno);
                txq = tid->txq;
@@ -2909,6 +2911,8 @@ void ath_tx_node_cleanup(struct ath_softc *sc, struct 
ath_node *an)
                if (!an->sta)
                        break; /* just one multicast ath_atx_tid */
+       rcu_read_unlock();
 #ifdef CONFIG_ATH9K_TX99

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